Water-dispersible core-shell resin which exhibits excellent sagging resistance in coatings

ABSTRACT

Disclosed are (1) a water-dispersible resin composition prepared by a process comprising; (I) reacting a silane monomer (A) having hydrolytic functional group and/or silanol group and polymerizable unsaturated bond with a vinyl monomer (B) in an aqueous medium to obtain a three-dimensionally crosslinked particulate polymer, (II) reacting the particulate polymer with the silane monomer (A) or a mixture of the silane monomer (A) and an allyl acrylate or allyl methacrylate (C), and (III) copolymerizing the resulting reaction product with a vinyl monomer component comprising a carboxyl-containing vinyl monomer (D), followed by neutralizing the carboxyl group; (2) a water-dispersible coating composition comprising a crosslinking agent and the water-dispersible resin composition prepared using a hydroxyl-containing vinyl monomer and another vinyl monomer as the vinyl monomer (B) in the step (I), and (3) a coating method characterized by using as a base coat composition a water-dispersible coating composition comprising the foregoing water-dispersible coating composition and a coloring pigment and/or metallic pigment in the two-coat method.

This application is a continuation of application Ser. No. 365,862 filedJun. 14, 1989, now abandoned.

The present invention relates to a novel water-dispersible resincomposition, a novel water-dispersible coating composition and a coatingmethod using the coating composition.

In recent years aqueous coating compositions have been increasingly usedin the coating industry to substitute water for a partial or wholequantity of organic solvent serving as a dissolving or dispersing mediumfrom viewpoints of savings of resources and prevention of pollutions.

However, aqueous coating compositions have the problem that thecomposition applied under high-humidity circumstances (e.g., a humidityof 80% or higher) is caused to flow before drying, giving an unevensurface (namely sagging).

Body panels of motor vehicles, two-wheel vehicles, electric products,etc. which must have a beautiful appearance have been frequently coatedby the so-called two-coat one-bake method or two-coat two-bake method(hereinafter both referred to as "two-coat method") to form a coatingexcellent in surface smoothness, distinctness-of-image gloss,weatherability and the like. The two-coat method comprises coating asubstrate such as a body panel with an organic solvent-dilutedthermosetting base coat composition containing a coloring pigment and/ormetallic pigment and applying an organic solvent-diluted thermosettingclear coating composition to the air-dried or heat-cured coating,followed by heat-curing the two coatings or the clear coating.Investigations are under way on use of aqueous coating compositions inthe two-coat method in place of base coat compositions containing alarge amount of organic solvent. Yet the use of aqueous coatingcompositions poses the foregoing serious problem.

It is known to prevent the sagging of applied aqueous coatingcomposition by using a sagging inhibitor capable of increasing theviscosity of coating composition and producing an preventive effect toan extent attainable by a protective colloid. The sagging inhibitorused, however, is unable to achieve a satisfactory effect.

An aqueous emulsion has been proposed for use in the two-coat one-bakemethod (U.S. Pat. No. 4,403,003). The proposed emulsion containscrosslinked polymer particles dispersed in water and each covered forprotection with a polymer, i.e. the so-called steric stabilizer, havinga hydrophilic portion and a lipophilic portion, and can be prepared, forexample, by subjecting to dispersion polymerization a polymerizablemonomer(s) in the presence of the steric stabilizer in an aqueousmedium. The emulsion is rendered thixotropic by the increase in thewater-solubility of hydrophilic portion due to neutralization offunctional group (e.g. carboxyl group) in the hydrophilic portion ofsteric stabilizer. The proposed emulsion is intended to prevent saggingby utilizing this phenomenon. The emulsion can prevent sagging moreeffectively than the sagging inhibitor, but can not sustain the effectfor a prolonged period of time. For example, when great shear force isexerted by agitation or transport through pipes, the emulsion isimparted reduced viscosity, lower thixotropy and increased fluidity,consequently providing a coating of lower sagging resistance. Moreover,the emulsion remains to be improved in the sagging resistance underhigh-humidity circumstances. The emulsion has the further problem setforth below. The emulsion is brought into contact with an organicsolvent when mixed therewith or applied as a base coat material followedby application of organic solvent-diluted clear coating composition inthe two-coat one-bake method. On contact therewith, the emulsion or thecoating film becomes swollen, tending to deteriorate the smoothness ofcoating surface.

An object of the present invention is to provide a novelwater-dispersible resin composition suitable for use as an aqueouscoating composition and free of the foregoing conventional problems.

Another object of the invention is to provide a novel water-dispersiblecoating composition capable of giving a coating which would cause no orlittle likelihood of sagging or metallic mottling if subjected to greatshear force or applied under high-humidity circumstances, and a coatingmethod using the coating composition.

These and other objects of the present invention will become apparentfrom the following description.

According to the present invention, there is provided awater-dispersible resin composition prepared by a process comprising thefollowing steps (I) to (III); (I) reacting a silane monomer (A) havinghydrolytic functional group and/or silanol group and polymerizableunsaturated bond with a vinyl monomer (B) in an aqueous medium to obtaina three-dimensionally crosslinked particulate polymer, (II) reacting theparticulate polymer with the silane monomer (A) and/or an allyl acrylateor allyl methacrylate (C), and (III) copolymerizing the resultingreaction product with a vinyl monomer component comprising acarboxyl-containing vinyl monomer (D), followed by neutralizing thecarboxyl group.

According to the invention, there are further provided awater-dispersible coating composition comprising a crosslinking agentand the water-dispersible resin composition prepared using ahydroxyl-containing vinyl monomer and another vinyl monomer as the vinylmonomer (B), and a coating method characterized by using as a base coatcomposition a water-dispersible coating composition comprising theforegoing water-dispersible coating composition and a coloring pigmentand/or metallic pigment in practicing the two-coat one-bake method orthe two-coat two-bake method.

In view of the foregoing prior art problems, we conducted extensiveresearch and found that the objects of the present invention can befulfilled by using (i) a water-dispersible resin composition prepared bychemically uniting a crosslinked particulate polymer in an emulsion witha polymer for stabilizing the crosslinked particulate polymer byspecific means, (ii) an aqueous coating composition comprising acrosslinking agent and the resin composition with the particulatepolymer having hydroxyl group and (iii) a coating method using theaqueous coating composition as a base coat composition in the two-coatmethod.

The present invention has been accomplished on the basis of this novelfinding.

The water-dispersible resin composition of the invention (hereinafterreferred to as "present composition") is an emulsion containingcore/shell-type polymer particles each composed of crosslinkedparticulate polymer as a core and a stabilizer polymer as a shell.

The present composition can be prepared by executing the following steps(I), (II) and (III).

(1) Step (I)

A silane monomer (A) having hydrolytic functional group and/or silanolgroup and polymerizable unsaturated bond (hereinafter referred to as"silane monomer (A)") is reacted with a vinyl monomer (B) in an aqueousmedium, giving an emulsion comprising a three-dimensionally crosslinkedparticulate polymer dispersed in water. The particulate polymerconstitutes the core.

(2) Step (II)

The particulate polymer in the emulsion is reacted with the silanemonomer (A) and/or an allyl acrylate or allyl methacrylate (C). In thisstep, presumably the silane monomer undergoes condensation reaction withthe functional group present on the surface of the particulate polymer,while the allyl acrylate or ally methacrylate is copolymerized with theunreacted polymerizable unsaturated bond remaining in the particulatepolymer. In any case, the reactions result in introduction ofpolymerizable unsaturated bond onto the surface of particulate polymer.

(3) Step (III)

In the emulsion formed by the reactions in the step (II), a vinylmonomer component comprising a carboxyl-containing vinyl monomer (D) iscopolymerized with the particulate polymer obtained in the step (II),followed by neutralizing the carboxyl group. The neutralized copolymeris one capable of stabilizing the dispersion of particulate polymer andconstituting the shell. In the step (III), the vinyl monomer componentis copolymerized with the polymerizable unsaturated bond formed from thesilane monomer and/or allyl acrylate or ally methacrylate on the surfaceof the particulate polymer resulting from the reactions in the step(II).

The steps (I) to (III) for preparing the present composition will bedescribed below in detail.

Step (I)

The silane monomer (A) to be used in the step (I) is a silane compoundhaving hydrolytic functional group and/or silanol group and containingpolymerizable unsaturated bond. Preferred silane monomers are compoundsrepresented by the formula (R₁)₃ --Si--X wherein R₁ is hydrolyticfunctional group and/or silanol group and X is a residue havingpolymerizable double bond and like polymerizable unsaturated bond,namely compounds having per molecule hydrolytic functional group and/orsilanol group numbering three in total and one residue having onepolymerizable unsaturated bond per molecule, all bonded to Si. Thesilane monomer to be used is one capable of forming a core wheninternally crosslinked.

Examples of hydrolytic functional groups represented by R₁ in theforegoing formula are alkoxy group having 1 to 12 carbon atoms,alkoxyalkoxy group having 3 to 15 carbon atoms, alkanoyloxy group having1 to 12 carbon atoms, etc. Examples of residues represented by X are##STR1## (wherein R₂ is hydrogen or CH₃ --, and n is an integer of 2 to10), etc.

Examples of the silane monomer (A) for use in the invention arevinyltrimethoxysilane, vinyltriethoxysilane,acryloxyethyltrimethoxysilane, methacryloxyethyltrimethoxysilane,acryloxypropyltrimethoxysilane, methacryloxypropyltrimethoxysilane,acryloxypropyltriethoxysilane, methacryloxypropyltriethoxysilane,vinyltris (β-methoxyethoxy)silane, etc. Among them, preferred silanemonomers are vinyltrimethoxysilane, γ-acryloxypropyltrimethoxysilane,γ-methacryloxypropyltrimethoxysilane, etc.

The vinyl monomer (B) to be copolymerized with the silane monomer (A) inan aqueous medium is a compound having at least one polymerizableunsaturated bond per molecule, and does not include the silane monomers.

Examples of the vinyl monomer (B) are as follows.

(1) Monoesters of acrylic acid or methacrylic acid with monohydricalcohol having 1 to 20 carbon atoms such as methyl acrylate, methylmethacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate,propyl methacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexylacrylate, 2-ethylhexyl methacrylate, cyclohexyl acrylate, cyclohexylmethacrylate, lauryl acrylate, lauryl methacrylate, etc.

(2) Aromatic vinyl monomers such as styrene, α-methylstyrene,vinyltoluene, etc.

(3) Hydroxyl-containing vinyl monomers: compounds having one hydroxylgroup and one polymerizable unsaturated bond per molecule among whichpreferable are monoesters of acrylic acid or methacrylic acid withdihydric alcohol having 2 to 10 carbon atoms such as 2-hydroxyethylacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl acrylate,hydroxypropyl methacrylate, etc.

(4) Carboxyl-containing vinyl monomers: compounds having at least onecarboxyl group and one polymerizable unsaturated bond per molecule suchas acrylic acid, methacrylic acid, crotonic acid, maleic acid, itaconicacid, etc.

(5) Glycidyl-containing vinyl monomers: compounds having one glycidylgroup and one polymerizable unsaturated bond per molecule such asglycidyl acrylate, glycidyl methacrylate, etc.

(6) Nitrogen-containing alkyl (C₁₋₂₀) acrylate such asdimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, etc.

(7) Amide compounds containing polymerizable unsaturated bond such asacrylic amide, methacrylic amide, dimethylacrylamide,N,N-dimethylpropylacrylamide, N-butoxymethylacrylamide, etc.

(8) Vinyl compounds such as vinyl acetate, vinyl chloride, etc.

(9) Nitrile compounds containing polymerizable unsaturated bond such asacrylonitrile, methacrylonitrile, etc.

(10) Diene compounds such as butadiene, isoprene, etc.

(11) Polyvinyl compounds having at least two polymerizable unsaturatedbonds per molecule such as ethylene glycol diacrylate, ethylene glycoldimethacrylate, triethylene glycol diacrylate, tetraethylene glycoldimethacrylate, 1,6-hexanediol diacrylate, divinylbenzene,trimethylolpropane triacrylate, etc. (Of said examples of polyvinylcompounds, preferable are those containing at least two unsaturatedbonds which are not widely different in reactivity from each other).

These vinyl monomers are usable singly or at least two of them can beused in mixture.

Of said examples of vinyl monomers, preferable are the monomers selectedfrom those shown above in (1), (2), (3) and (11).

In the step (I), the particulate polymer is prepared by copolymerizingthe silane monomer (A) and the vinyl monomer (B) in an aqueous medium togive a crosslinked particulate polymer as described above. Thiscopolymerization can be done, for example, by the following conventionalemulsion polymerization methods.

(i) A mixture of monomers (A) and (B) is added dropwise in an atmosphereof inert gas to an aqueous medium being stirred and consisting of waterand a surfactant to copolymerize the monomers at a specific temperature.

(ii) An emulsion of monomers (A) and (B) dispersed in an aqueous mediumis added dropwise to water being stirred to copolymerize the monomers ata specific temperature.

(iii) A small quantity of monomer or monomers (singly or in mixture) issubjected to seed polymerization and the resulting polymer is subjectedto emulsion polymerization by the method (i) or (ii).

Among the foregoing methods, the method (iii) is suitable because it iscapable of reducing the particle size and improving the saggingresistance and surface smoothness.

It is preferred to conduct these emulsion polymerizations in thepresence of a radical polymerization initiator.

The monomers are used in the steps (I) and (III) in the followingproportions. The total monomers are used in the step (I) in an amount ofabout 30 to about 95% by weight, preferably about 60 to about 90% byweight, based on the combined weight of monomers for use in the steps(I) and (III). The total monomers are used in the step (III) in anamount of about 70 to about 5% by weight, preferably about 40 to about10% by weight, based on the combined weight of monomers for use in thesteps (I) and (III). Used in the step (I) are the silane monomer (A) andvinyl monomer (B), more specifically about 0.5 to about 20% by weight,preferably about 1 to about 10% by weight, of the silane monomer (A) andabout 99.5 to about 80% by weight, preferably about 99 to about 90% byweight, of the vinyl monomer (B), based on the combined weight ofmonomers (A) and (B). In the step (III), the monomer component includesthe carboxyl-containing vinyl monomer (D) in an amount of about 1 toabout 50% by weight, preferably about 3 to about 30% by weight, based onthe weight of vinyl monomer component. The vinyl monomer component mayfurther contain about 10% by weight or less of a silane monomer and/orpolyvinyl compound exemplified above in (11) as the vinyl monomer foruse in the step (I).

The surfactant to be used for the emulsion polymerization in the step(I) can be either anionic or nonionic. Examples of useful anionicsurfactants are an alkali metal salt or ammonium salt of long-chainalkyl sulfate, long-chain alkyl sulfonate or long-chain alkylsulfosuccinate; alkali metal salt of alkyl disulfate; alkali metal saltor ammonium salt of polyoxyethylene alkylphenyl ether sulfate; alkalimetal salt or ammonium salt of alkyldiphenyl ether disulfonate; alkalimetal salt or ammonium salt of dialkyl sulfosuccinate; etc. Examples ofuseful nonionic surfactants are polyoxyethylene alkylphenyl ether,polyoxyethylene alkyl ether, polyoxyethylene long-chain carboxylate,etc. These surfactants are usable singly or at least two of them can beused in mixture. The amount of the surfactant to be used is about 0.05to about 10 parts by weight, preferably about 0.1 to about 5 parts byweight, more preferably about 0.5 to about 3 parts by weight, per 100parts of the total monomers to be used in the step (I).

Suitable examples of the radical polymerization initiator for use in thestep (I) include peroxide compounds such as ammonium persulfate, sodiumammonium persulfate, potassium ammonium persulfate, potassiumpersulfate, sodium pyrosulfite, t-butyl hydroperoxide, cumenehydroperoxide, etc. These radical polymerization initiators are usablesingly or at least two of them can be used in mixture. The amount of thepolymerization initiator to be used is about 0.1 to about 10 parts byweight, preferably about 0.2 to about 5 parts by weight, per 100 partsof the total monomers for use in the step (I).

In the step (I), the two monomers are copolymerized at a temperature ofabout 90° C. or lower, preferably about 25 to about 90° C.

It is desirable to carry out the copolymerization in the step (I) usingthe two monomers in the total concentration of about 10 to about 60% byweight.

The emulsion obtained in the step (I) comprises the particulate polymerformed by three-dimensionally crosslinking the silane monomer (A) andthe vinyl monomer (B) and dispersed in an aqueous medium. Thecrosslinking reaction of two monomers occurs together with thecopolymerization of polymerizable unsaturated bonds contained in the twomonomers and the condensation reaction of hydrolytic functional group inthe silane monomer (A) and/or of silanol group therein, and optionallywith the reaction of hydroxyl group which may be present in the vinylmonomer (B) with the silane monomer (A). The reactions presumably givepolymer particles composed of three-dimensionally crosslinked bonds,i.e. --C--C-- bonds formed from polymerizable unsaturated bonds and--Si--O--Si--bonds formed from the silane monomer (A). Also presumableis that the hydrolytic functional group and/or silanol group derivedfrom the silane monomer (A) is attached as unreacted to the surface ofthe polymer particles. The particle size of the polymer particlesobtained in the step (I) varies according to the kind and amount ofsurfactant and the like and the polymerization method, but ranges fromabout 10 to about 500 nm, preferably about 30 to about 300 nm.

Step (II)

The silane monomer (A) and/or an allyl acrylate or allyl methacrylate(C) is reacted with the particulate polymer in the emulsion obtained inthe step (I) to introduce the polymerizable unsaturated bond onto thesurface of particulate polymer. The particulate polymer produced in thestep (II) with the polymerizable unsaturated bond introduced therein ishereinafter referred to as "unsaturated particulate polymer".

The silane monomer (A) is reacted with the unreacted hydrolyticfunctional group and/or silanol group attached to the surface of theparticulate polymer obtained in the step (I) to introduce thepolymerizable unsaturated bond onto the surface thereof.

Stated more specifically, the silane monomer (A) is incorporated intothe emulsion produced in the step (I), and the resulting emulsion isleft to stand at a temperature of, e.g. about 20 to about 90° C.,whereby the silane monomer (A) becomes adsorbed onto the surface of theparticulate polymer in the emulsion while the foregoing reactionproceeds to introduce the polymerizable unsaturated bond onto thesurface thereof.

The ratio of the particulate polymer and the silane monomer (A) to beused in the step (II) is not specifically limited. Yet the silanemonomer (A) is used in the step (II) in an amount of preferably about0.5 to about 2 moles per mole of the silane monomer (A) used in the step(I) (usually about 50 to about 200 parts by weight of the former per 100parts by weight of the latter). A lesser amount of the former used tendsto reduce the amount of polymerizable double bond introduced and toimpair the stability of the present composition, whereas a larger amountthereof is apt to cause coagulation of particulate polymer.

The monomer (C) is an allyl acrylate or allyl methacrylate. Of the allyldouble bond and the acrylic or methacrylic double bond present in themonomer (C), the acrylic or methacrylic bond is more readily polymerizedthan the former. When the monomer (C) is incorporated into the aqueousdispersion obtained in the step (I) and the dispersion is heated toabout 40 to about 90° C., the acrylic or methacrylic double bond in themonomer (C) is copolymerized with the unreacted monomer contained in theparticulate polymer of the aqueous dispersion, whereby the lesspolymerizable allyl double bond presumably remains in an unreacted stateon the surface of the particulate polymer. A preferred amount of themonomer (C) to be used is about 0.5 to about 5 parts by weight per 100parts by weight of the particulate polymer.

In the event the monomers (A) and (C) are conjointly used, the foregoingreactions occur concurrently.

Conjoint use of monomers (A) and (C) is preferred to accomplish theobjects of the invention. The proportions of the particulate polymer andthese monomers are as shown above.

Step (III)

The unsaturated particulate polymer obtained in the step (II) iscopolymerized with a vinyl monomer component including acarboxyl-containing vinyl monomer (D). Subsequently the carboxyl groupis neutralized to form a shell.

The vinyl monomer component comprises a carboxyl-containing vinylmonomer (D) and another vinyl monomer. Suitable examples of the vinylmonomer component include the foregoing examples of the vinyl monomer(B) to be copolymerized with the silane monomer in the step (I). Morespecifically, usable as the monomer (D) is at least one monomer selectedfrom the examples set out above in (4) as the carboxyl-containing vinylmonomer. Among the examples (4), acrylic acid, methacrylic acid and thelike are preferred. The other vinyl monomer of the vinyl monomercomponent is a compound other than the carboxyl-containing vinyl monomerand having one polymerizable unsaturated bond per molecule. Usable assuch monomers are those exemplified above in (1) to (3) and (5) to (10).The polyvinyl compounds shown above in (11) and silane monomer (A) canbe conjointly used.

The carboxyl-containing vinyl monomer (D) and the other vinyl monomerfor the vinyl monomer component are used in the following proportions.The proportions are about 1 to about 50% by weight, preferably about 3to about 30%by weight, of the vinyl monomer (D) and about 99 to about50% by weight, preferably about 97 to about 70% by weight of, the othervinyl monomer, based on the total weight of two monomers. The vinylmonomer component is used in an amount of about 70 to about 5% byweight, preferably about 40 to about 10% by weight, based on thecombined weight of the silane monomer (A) and the vinyl monomer (B) usedas described above in the step (I) (namely the particulate polymerobtained in the step (I)), and the vinyl monomer component. When asilane monomer and polyvinyl compound are conjointly used in the step(III), a preferred amount of these components is 10 parts by weight orless per 100 parts by weight of the vinyl monomer component.

The copolymerization of the vinyl monomer component and the particulatepolymer in the step (III) can be carried out by the emulsionpolymerization method (i) or (ii) stated hereinbefore. For thiscopolymerization, the vinyl monomer component is incorporated into theemulsion of the unsaturated particulate polymer obtained in the step(II) and the mixture is reacted at a temperature of about 50 to about90° C. It is preferred to conduct this reaction in the presence of theforegoing radical polymerization initiator when so required.

The step (III) is performed to unite chemically the unsaturatedparticulate polymer (core) obtained in the step (II) with the linearcopolymer (shell) composed of the vinyl monomer component.

Stated more specifically, the vinyl monomer component is copolymerizedwith the bonds attached to the surface of the unsaturated particulatepolymer obtained in the step (II), namely the polymerizable unsaturatedbond of the silane monomer and/or the polymerizable unsaturated bondremaining in the allyl acrylate or methacrylate copolymerized with theunreacted monomer contained in the particulate polymer, whereby thecopolymer of the vinyl monomer component (shell) is chemically unitedwith the surface of particulate polymer (core) obtained in the step (I).When the silane monomer (A) and the polyvinyl compound (11) areconjointly used in the step (III), the linear copolymer is a reticulateone which can be chemically united with the particulate polymer.

In the step (III), the unsaturated particulate polymer is copolymerizedwith the vinyl monomer component to form a shell after which thecarboxyl group of the shell is neutralized. The neutralization isconducted by incorporating a neutralizing agent into the reactionsystem. Examples of useful neutralizing agents are sodium hydroxide,potassium hydroxide and like inorganic alkali substances, ammonia andamine compounds such as monoethanolamine, ethylamine, dimethylamine,diethylamine, triethylamine, propylamine, dipropylamine, isopropylamine,diisopropylamine, triethanolamine, butylamine, dibutylamine,2-ethylhexylamine, ethylenediamine, propylenediamine,methylethanolamine, dimethylethanolamine, diethylethanolamine,morpholine, etc.

The neutralization reaction is performed at room temperature or at anelevated temperature, giving the present composition. The particulatepolymer eventually obtained in the step (III) comprises the core formedin the step (I) and the shell attached thereto and made hydrophilic byneutralization. The thus obtained particulate polymer has a particlesize about 10 to about 100% larger than the particulate polymer obtainedin the step (I) as the core.

The present composition thus prepared is an aqueous emulsion ofcore/shell-type particulate polymer prepared by carrying out the steps(I) to (III). The content of resin solids in the emulsion can besuitably varied but is usually in the range of about 20 to about 50% byweight.

The present composition is suitable for use as a usual coatingcomposition, a coating composition for finishing paper sheets,adhesives, etc. Before use, a coloring pigment, extender pigment,metallic pigment and the like can be incorporated into the presentcomposition.

The present composition is preferably mixed with a conventional aqueouscoating composition to make the aqueous composition thixotropic forprevention of sagging. Examples of the conventional aqueous coatingcompositions to be mixed are water-soluble or water-dispersible coatingcompositions predominantly containing an acrylic resin, vinyl resin,alkyd resin, polyester resin or the like. In this case, a preferredamount of the present composition included in the aqueous coatingcomposition is about 10 to about 300 parts by weight, calculated assolids, per 100 parts of the resin solids in the aqueous composition.

According to the invention, a water-dispersible coating compositionhaving excellent properties can be obtained by mixing a crosslinkingagent with the water-dispersible resin composition of the inventionprepared using as the vinyl monomer (B) any of the hydroxyl-containingvinyl monomers exemplified above in (3) and another vinyl monomer.

The water-dispersible coating composition of the invention can beprepared by adding a crosslinking agent to the present compositionformed by execution of the steps (I) to (III). For this preparation, itis critical that a three-dimensionally crosslinked particulate polymerbe prepared in the step (I) by reacting in an aqueous medium the silanemonomer (A), the hydroxyl-containing vinyl monomer (3) and the vinylmonomer (B) other than said monomers. The hydroxyl-containing vinylmonomer (3) is used to introduce, onto the surface of particulatepolymer, hydroxyl group acting as functional group reactive with thecrosslinking agent.

The vinyl monomer (B) to be copolymerized with the silane monomer (A)and the hydroxyl-containing vinyl monomer (3) in an aqueous medium is acompound having at least one polymerizable unsaturated bond per moleculeand include the compounds indicated above in (1), (2) and (4) to (11)and other than the silane monomer (A) and the hydroxyl-containing vinylmonomer (3). The vinyl monomer (B) is used to improve the waterresistance, adhesion between layers, stability of aqueous dispersion,etc.

The monomers for use in the step (I) are the silane monomer (A),hydroxyl-containing vinyl monomer (3) and other vinyl monomer (B) whichare used in the following proportions. The proportions are about 0.5 toabout 20% by weight, preferably about 1 to about 10% by weight, of thesilane monomer (A), about 1 to about 30% by weight, preferably about 2to about 20% by weight, of the hydroxyl-containing vinyl monomer (3),about 98.5 to about 50% by weight, preferably about 97 to about 70% byweight, of the vinyl monomer (B), based on the combined weight of totalmonomers.

The hydroxyl-containing vinyl monomer (3) may be used in addition to thecarboxyl-containing vinyl monomer (D) for the vinyl monomer component inthe step (III) in preparation of the present water-dispersible coatingcomposition. The amount of the vinyl monomer (3) to be used is about 30%by weight or less, preferably about 25% by weight or less, based on theweight of the vinyl monomer component. The compounds exemplified aboveas the monomer (B) (other than the monomers (3), (4) and (11)) areusable in addition to the vinyl monomer (D), or alternatively the vinylmonomer (D) and the vinyl monomer (3) for the vinyl monomer component inthe step (III). The amount of such compound for use is about 99 to about20% by weight, preferably about 97 to about 45% by weight, based on theweight of the vinyl monomer component.

The vinyl monomer component to be used in the step (III) as stated abovemay include the silane monomer (A) and/or any of polyvinyl compoundsshown above in (11) as the vinyl monomer (B) each in a ratio of about10% by weight or less, based on the weight of the vinyl monomercomponent.

The crosslinking agent present in the water-dispersible coatingcomposition is one capable of dissolving or dispersing uniformly in thepresent composition and crosslinkable when heated with the hydroxylgroup of the particulate polymer eventually obtained in the step (III)to form a cured coating film.

Examples of useful crosslinking agents are melamine resin,benzoguanamine resin, urea resin and like amino resins as methylolated,these resins as etherified with alkyl, phenolformaldehyde resin, blockpolyisocyanate compounds, etc. While a water-soluble or hydrophobiccrosslinking agent is usable, a hydrophobic one is preferably used toimprove the amenability to coating operation, storage stability,humidity resistance and the like.

For example, suitable hydrophobic melamine resins are those having asolvent dilution ratio of about 20 to about 0.1, preferably about 18 toabout 0.1 and a weight-average molecular weight of about 700 to about4,000, preferably about 800 to about 3,000. The solvent dilution ratioused in the invention is an index indicating the solubility of melamineresin in hydrophilic solvents. The lower the ratio, the more hydrophobicis the resin. The solvent dilution ratio is determined by the followingmethod. Two grams of melamine resin is placed into a .increment.50-ccbeaker, which is then placed on paper bearing a print of No. 5 type.Subsequently, at 25° C. a mixture of water and methanol (35/65 in weightratio) is added dropwise to the resin with stirring until the printbecomes illegible. The amount (cc) thus added is divided by the amountof the melamine resin to obtain a value (cc/g) as the ratio.

The hydrophobic melamine resin is not limited specifically insofar as itfulfills the solvent dilution ratio and molecular weight requirements.The resin is usable as etherified variously, for example, as modifiedwith at least one of methyl alcohol, ethyl alcohol, isopropyl alcohol,n-butyl alcohol, isobutyl alcohol, octyl alcohol, 2-ethylhexyl alcohol,benzyl alcohol, etc. According to the invention, it is suitable to usethe resin as modified with an alcohol having at least four carbon atoms,more preferably four to seven carbon atoms. The amount of ether groupsin the melamine resin, although not limited specifically, is suitablyabout 5 moles or less, preferably about 1.5 to about 3 moles, pertriazine ring. Further as to the functional groups such as amino, iminoand methylol, the kind and amount of remaining functional groups are notlimited specifically provided that the foregoing solvent dilution ratioand molecular weight requirements are satisfied. Usually, however, theamount of imino groups (inclusive of amino groups), as well as ofmethylol groups, is about 0.2 to about 2.0 moles, preferably about 0.5to about 1.5 moles, per triazine ring.

The hydrophobic crosslinking agent is preferably mixed with awater-soluble resin before blending with the present composition so thatthe surface of hydrophobic crosslinking agent (usually in a particulateform) is coated with the resin to improve the amenability to coatingoperation (prevention of sagging, etc.), storage stability and the like.

Water-soluble resins useful for this purpose are those having introducedtherein a quantity of hydrophilic groups, such as carboxyl (--COOH),hydroxyl (--OH), methylol (--CH₂ OH), amino (--NH₂), sulfo (--SO₃ H) orpolyoxyethylene bond (CH₂ CH₂ O_(n)). Examples of such resins areacrylic resin, alkyd resin, epoxy resin and the like. The most typicalof such water-soluble resins are those having carboxyl groups introducedtherein, neutralized to an alkali salt and thereby made soluble inwater.

Suitable proportions of the hydrophobic crosslinking agent andwater-soluble resin are about 20 to about 100 parts by weight,preferably about 28 to about 80 parts by weight, of the latter per 100parts by weight of the former calculated as solids. The two componentscan be mixed by any suitable method, for example by mixing together thetwo components using a stirrer, homomixer or the like to provide ahomogeneous blend. When required, a small amount of hydrophilic solvent,such as alcohol solvent or ether solvent, can be added to the mixture inthis step. Next, deionized water is added in small portions to themixture in about 0.5 to about 5 times the combined amount by weight ofthe two components while vigorously stirring the mixture, whereby thecrosslinking agent is made into a milky white one or colored aqueousdispersion. The dispersion contains particles about 0.05 to about 0.5 μmin mean particle size.

The amount of the crosslinking agent to be used is about 10 to about 50%by weight, preferably about 15 to about 40% by weight, based on thetotal weight of the agent and the resin solids in the presentcomposition (in the event another resin or aqueous coating compositionis used along with the present composition, the resin solids containedtherein are added).

The water-dispersible coating composition of the invention may contain ametallic pigment and/or coloring pigment. Examples of useful metallicpigments are aluminum flake, copper bronze flake, micaceous iron oxide,mica flake, metallic oxide-coated macaceous iron oxide, metallicoxide-coated mica flake, etc. Examples of useful coloring pigments aretitanium dioxide, iron oxide, chromium oxide, lead chromate, carbonblack and like inorganic pigments, Phthalocyanine Blue, PhthalocyanineGreen, Carbazole Violet, Anthrapyrimidine Yellow, Flavanthrone Yellow,Isoindoline Yellow, Indanthrone Blue, Quinacridone Violet and likeorganic pigments. Dyes are also usable as the coloring pigment.

To facilitate the formation of coating, the water-dispersible coatingcomposition of the invention preferably contains a water-soluble orwater-dispersible resin predominantly containing acrylic resin, vinylresin, alkyd resin, polyurethane resin, polyester resin or the like. Theamount of such resin to be used is about 20 to about 1,000 parts byweight per 100 parts by weight of the resin solids in the presentcomposition. When required, the present composition may further containa defoaming agent, thickener, fungicide and the like.

The coating composition of the invention is suitable for use as a usualtop coat composition, a coating composition for finishing paper sheetsand other purposes.

The coating formed from the water-dispersible coating composition of theinvention can be cured by heating usually at a temperature of about 120to about 180° C.

The coating method of the present invention will be described below.

The coating method of the invention is characterized in that awater-dispersible coating composition comprising the coating compositionof the invention and a coloring pigment and/or metallic pigment is usedas a base coat composition in the two-coat method.

The water-dispersible coating composition for use as a base coatcomposition is prepared by dispersing the coloring pigment and/ormetallic pigment in the coating composition of the invention.

The conventional clear coating composition to be used in the coatingmethod of the invention can be any of clear coating compositionscommonly used in the two-coat method. Stated more specifically, usefulclear coating compositions are those comprising a base resin and acrosslinking agent as essential components and an organic solvent fordissolving or dispersing the components, and capable of forming atransparent coating film. Examples of useful base resins are alkydresin, acrylic resin, polyester resin, silicone polyester resin,urethane resin, etc. which are required to have functional groupcrosslinkable with a crosslinking agent. Examples of useful crosslinkingagents are melamine resin, benzoguanamine resin, urea resin and likeamino resins as methylolated, these resins as etherified with loweralkyl, aliphatic dibasic acid, polyisocyanate compounds, blockpolyisocyanate compounds, etc.

The clear coating composition for use herein may contain a coloringpigment and/or metallic pigment insofar as the pigment does not impairtransparency.

Useful clear coating compositions may be of a solution type, dispersiontype, nonaqueous dispersion type or highly solid type. A powder-typecomposition is also usable.

Substrates suitable for the coating method of the invention includeouter panels of automobiles, buses, trucks, motorcycles and the like.Also usable are outer plates of electric appliances and like substrateswhich must have a beautiful finishing appearance. These substrates maybe those coated with a primer or an intercoat composition.

In use as a base coat composition in the two-coat one-bake method, thecoating composition of the invention is preferably adjusted to a solidscontent of about 10 to about 40% by weight and to a viscosity of about500 to about 4,000 cps/6 rpm (B-type viscometer) by adding deinoizedwater and, when required, additives such as a thickener and defoamingagent as in a usual process. The base coat composition is applied to asubstrate as by spray coating or the like to form a coating of about 10to about 50 μm in dry thickness and then dried in air or hot air untilthe volatile content of the coating reduces to about 25% by weight orlower, preferably about 15% by weight or lower. A clear coatingcomposition with a viscosity adjusted to about 10 to about 30 sec (fordcup #4/20° C.) is then applied as by electrostatic spray coating or thelike to the resulting coating to a dry thickness of about 15 to about 70m, preferably about 25 to about 40 μm. Subsequently, the coatedsubstrate is set in a usual manner and then heated to about 120 to about180° C. to cure the coatings. In application, the coating composition ofthe invention is highly amenable to the coating operation, readilyforming a beautiful coating.

In case of the two-coat two-bake coating system, the coating method ofthe invention is carried out in the same manner as above except that thebase coating is heated at the foregoing temperature and cured beforeapplication of clear coating composition.

In the coating method of the invention, the base coating and the clearcoating are cured chiefly by crosslinking reaction caused by the actionof crosslinking agent in the coatings.

The present composition is characterized in that the core/shell polymerparticles in the emulsion has the core united with the shell by aspecific method. And the present coating composition is characterized inthat it comprises the present composition and a crosslinking agent. Thefollowing technical advantages result from said features of theinvention.

(1) The polymerizable unsaturated bond can be easily introduced into thecore in the step (II) and the copolymerization can be performedconveniently in the step (III), with the result that the core and theshell can be chemically united together with ease.

(2) In the step (III), the carboxyl group in the shell is neutralizedwhereupon the polymer molecules in the shell are partially or entirelydissolved and diffused in the aqueous medium, making the compositionthixotropic. With the thixotropy, the present composition exhibitslittle or no decrease of viscosity when subjected to great shear forcebecause the core is chemically united with the shell as stated above. Inconsequence, the present coating composition comprising the presentcomposition and a crosslinking agent gives a coating which would beunlikely to sag even when applied after submission to great shear force.

(3) The present coating composition provides a coating which would beunlikely to sag when applied under high-humidity circumstances for thereason set out above in (2).

(4) The core/shell portions of the particles in the present coatingcomposition swell due to contact with an organic solvent but in a lesserdegree than in conventional coating compositions. Accordingly thepresent coating composition undergoes no undesirable change inproperties (such as storage stability, mechanical stability, circulationstability, etc.) if the coating composition contains an organic solventor an organic solvent-containing component (e.g. pigment paste and thelike). Moreover, an organic solvent-diluted coating composition can beapplied over the surface of uncured (wet) coating formed from thepresent coating composition.

In comparison on swelling due to organic solvent, the present coatingcompositions exhibited a three-fold or less swelling in most cases,whereas coating compositions with the core and shell not chemicallyunited generally showed a four-fold or more swelling. In the comparison,ethanol was used as an organic solvent and the particle size wasmeasured by "COULTER N-4" (sub-micron particle analyzer manufactured byCoulter Electronics Inc.).

(5) Since the core of the particles in the present coating compositioncontains a silane monomer and has silanol group on the surface thereof,the core/shell particles have cohesion sufficient to remain free ofsedimentation. This factor, coupled with the characteristic stated abovein (2), contributes to prevention of sagging.

(6) The present coating composition because of the crosslinking agentpresent therein can form a coating which, when heated, becomesthree-dimensionally crosslinked on reaction of the crosslinking agentwith the hydroxyl group in the composition. Consequently the formedcoating is excellent in water resistance, chemical resistance,weatherability, physical properties, etc.

With these advantages, the present coating composition is significantlysuitable for use as an aqueous base coat composition in the two-coatmethod. Therefore the coating method of the invention has the advantagesof forming a coating excellent in surface smoothness, adhesion,particularly adhesion between layers, gloss, metallic effect, waterresistance, etc.

The present invention will be described below in greater detail withreference to the following Examples and Comparison Examples in which theparts and the percentages are all by weight.

I. PREPARATION OF PRESENT COMPOSITION Examples 1 to 6 and ComparisonExamples 1 to 3

Water-dispersible resin compositions were prepared using the monomersshown below in Table 1.

Step (I)

A flask was charged with 120 parts of deionized water, which was thenheated to 80 to 85° C. Two parts of the first pre-emulsion shown inTable 1 was added dropwise with stirring. After the mixture was aged for20 minutes, the remaining portion of the first pre-emulsion was addeddropwise at a constant rate and the same temperature over a period of 3hours, giving an aqueous dispersion of three-dimensionally crosslinkedparticulate polymer as a core.

Step (II)

A silane monomer was added dropwise to the aqueous dispersion uponcompletion of addition of first pre-emulsion. The mixture was maintainedat 80 to 85° C. for 1 hour to react the silane monomer on the surface ofthe core.

Step (III)

Fifty parts of deionized water was added and the second pre-emulsionshown in Table 1 was added dropwise at a constant rate at 80 to 85° C.over a period of 1 hour.

Subsequently a solution of 0.06 part of ammonium persulfate in 18 partsof deionized water was added dropwise at the same temperature over aperiod of 1 hour. The mixture was maintained at the same temperature for1 hour and rapidly cooled to room temperature after which deionizedwater was added to provide a solids content of 30%. The thus obtainedparticulate polymer was insoluble in an organic solvent. Then deionizedwater was added and the mixture was neutralized withdimethylethanolamine and adjusted to a 20% solids content, producing acore/shell type water-dispersible resin composition.

                                      TABLE 1                                     __________________________________________________________________________                     Example                 Comp. Ex.                                             1   2   3   4   5   6   1   2   3                            __________________________________________________________________________    Step (I)                                                                      First Pre-emulsion (*1)                                                       γ-Methacryloxy propyl trimethoxy                                                         2   4       2   2   2   2                                    silane                                                                        Vinyl trimethoxy silane  2                                                    Styrene          20  20  20  20  20  20  20  20                               Methyl methacrylate                                                                            23  21  23  23  23  23  23  22  41                           Butyl acrylate   40  40  40  40  40  40  40  40  40                           Methyl acrylate  15  15  15  15  15  15  15  15  15                           Allyl acrylate                               3                                Ethylene glycol dimethacrylate                   2                            MGA (*2)                                         2                            Ammonium salt of polyoxyethylene                                                               1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5                          nonylphenyl ether sulfate                                                     Ammonium persulfate                                                                            0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4                          Deionized water  45  45  45  45  45  45  45  45  45                           Step (II)                                                                     γ-Methacryloxy propyl trimethoxy                                                         2   4       2   2   2   --  --  --                           silane                                                                        Vinyl trimethoxy silane  2                                                    Step (III)                                                                    Second Pre-emulsion (*1)                                                      Acrylic acid     16  16  16  16  16      16  16  16                           Methacrylic acid                     20                                       Styrene          12  12  12  12  12  12  12  12  20                           Methyl methacrylate                                                                            16  16  16  16  16  16  16  16  16                           Butyl acrylate   40  40  40  40  40  36  40  40  40                           Ethyl acrylate   16  16  16  16  16  16  16  16  16                           Ammonium salt of polyoxyethylene                                                               0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3                          nonylphenyl ether sulfate                                                     Ammonium persulfate                                                                            0.02                                                                              0.02                                                                              0.02                                                                              0.02                                                                              0.02                                                                              0.02                                                                              0.02                                                                              0.02                                                                              0.02                         Deionized water  12  12  12  12  12  12  12  12  12                           First Pre-emulsion/                                                                            80/20                                                                             80/20                                                                             80/20                                                                             70/30                                                                             90/10                                                                             80/20                                                                             80/20                                                                             80/20                                                                             80/20                        Second Pre-emulsion                                                           (Weight ratio of monomers) (*3)                                               Before neutralization                                                         Particle size (nm) (*5)                                                                        110 134 130 138 95  108 105 110 95                           pH               2.5 2.4 2.3 2.2 2.9 2.2 2.5 2.9 2.4                          After neutralization (*4)                                                     Particle size (nm) (*5)                                                                        112 135 132 144 95  107 106 112 96                           pH               7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5                          __________________________________________________________________________

The symbols *1 to *5 used in Table 1 designate the following.

(*1) Pre-emulsion: An emulsion prepared by uniformly dispersing themixture of the listed components by a high-speed stirrer.

(*2) MGA: A compound represented by the formula ##STR2## (*3) Ratio byweight of polymerizable monomers contained in each pre-emulsion.

(*4) Dimethylethanolamine was used for neutralization.

(*5) Measured by "COULTER N-4" (sub-micron particle analyzermanufactured by Coulter Electronics Inc.).

The water-dispersible resin compositions prepared in the Examples andComparison Examples were tested singly or in mixture with a usualcold-curing emulsion coating composition in a ratio by weight of 50/50calculated as solids.

The amenability to coating operation was expressed in terms of saggingresistance, which was determined in the following manner. The coatingcomposition (specimen) was applied by a spray coater to a slate sheet (5mm in thickness) vertically disposed and having a hole, 1 cm indiameter, to form a coating of 20 μm thickness when dried. The coatedsheet was dried at room temperature, and the length (unit: mm) ofsagging of the applied composition formed downward from the hole wasmeasured.

The mechanical stability is the result obtained by testing singly thewater-dispersible resin compositions prepared in the Examples andComparison Examples, and was evaluated in terms of viscosity stabilityand TI value-retaining percentage (to be clarified below). The viscositystability wa given by the following procedure. A viscosity (η₀) wasdetermined by a Brookfield viscometer (rotating the agitating blade at 6rpm) after the composition was left to stand for 24 hours at roomtemperature. Thereafter the composition was stirred at room temperaturefor 3 minutes by a stirrer (rotating the agitating blade at about 800rpm). Then after standing for 5 minutes, a viscosity (η₁) was measuredin the same manner as above. The ratio of η₁ /η₀ was calculated topresent the viscosity stability. The TI value-retaining percentage wasprovided by the following method. After the composition was left tostand at room temperature for 24 hours, viscosities η₆ and η₆₀ weredetermined by Brookfield viscometer rotating the agitating blade at 6rpm and 60 rpm, respectively and a viscosity ratio TI₀ (=η₆ /η₆₀) wascalculated. Subsequently the composition was stirred at room temperaturefor 3 minutes by a stirrer rotating the agitating blade at about 800rpm. After standing for 5 minutes, viscosities η₆ and η₆₀ were measuredby a Brookfield viscometer rotating the agitating blade at 6 rpm and 60rpm, respectively to obtain a viscosity ratio TI₁ (=η₆ /η₆₀). Apercentage was given by the following equation: (TI₁ /TI₀)×100 (%)(which is termed herein TI value-retaining percentage).

The usual cold-curing emulsion coating composition to be mixed with thecomposition (specimen) was prepared by the following process. Maleinized1,2-vinyl polybutadiene having a number-average molecular weight ofabout 3,000 and an acid value of about 100 was dissolved in ethyleneglycol monobutyl ether to a solids content of 74%. A 216 g portion ofthe solution was neutralized with ammonia to 0.95 equivalent, anddissolved in 780 g of water. Added thereto was 414 g of n-butylmethacrylate, and the mixture was thoroughly stirred to obtain anemulsion to which a solution of 1 g of ammonium persulfate in 20 g ofwater was added. The mixture was heated and maintained at 80° C. for 2hours, giving the contemplated emulsion coating composition.

Table 2 below shows the results.

                                      TABLE 2                                     __________________________________________________________________________               Example           Comp. Ex.                                                   1  2  3  4  5  6  1  2  3                                          __________________________________________________________________________    Amenability to                                                                coating operation                                                             Specimen singly used                                                          Humidity 70%                                                                             2.5                                                                              1.8                                                                              2.5                                                                              2.5                                                                              2.3                                                                              2.4                                                                              4.5                                                                              5.3                                                                              4.7                                        Humidity 90%                                                                             3.7                                                                              3.0                                                                              4.0                                                                              4.0                                                                              3.6                                                                              3.5                                                                              7.4                                                                              8.6                                                                              7.9                                        Specimen in mixture                                                           Humidity 70%                                                                             3.4                                                                              2.9                                                                              3.8                                                                              4.1                                                                              3.7                                                                              3.9                                                                              7.0                                                                              7.0                                                                              6.8                                        Humidity 90%                                                                             6.0                                                                              5.5                                                                              6.7                                                                              6.9                                                                              6.0                                                                              6.8                                                                              11.5                                                                             13.4                                                                             11.3                                       Mechanical                                                                    stability                                                                     Viscosity stability                                                                      0.96                                                                             0.98                                                                             0.93                                                                             0.91                                                                             1.0                                                                              0.98                                                                             0.80                                                                             0.83                                                                             0.85                                       TI-value retaining                                                                       85 91 80 81 97 88 60 65 67                                         percentage (%)                                                                __________________________________________________________________________

II. PREPARATION OF WATER-DISPERSIBLE COATING COMPOSITION OF THEINVENTION II-1 Preparation of Present Composition

The present compositions were prepared using the monomers shown below inTable 3.

Step (I)

Deionized water (120 parts) was placed into a flask and heated at 80 to85° C. The first pre-emulsion (2 parts) as shown in Table 3 below wasadded dropwise with stirring. After the mixture was aged for 20 minutes,the remaining amount of first pre-emulsion was added dropwise at aconstant rate and the same temperature over a period of 3 hours toobtain an aqueous dispersion of three-dimensionally crosslinkedparticulate polymer as a core.

Step (II)

On completion of addition of first pre-emulsion, a silane monomer and anallyl methacrylate were added dropwise. The mixture was maintained at 80to 85° C. for 1 hour to react the silane monomer and the allylmethacrylate on the surface of the core.

Step (III)

Fifty parts of deionized water was charged and the second pre-emulsionshown below in Table 3 was added dropwise at 80 to 85° C. at a constantrate over a period of 1 hour.

The mixture was maintained at the same temperature for 1 hour andrapidly cooled to room temperature. Deionized water was added to achievea solids content of 30%. The obtained particulate polymer was insolublein an organic solvent. Deionized water was added and the mixture wasneutralized with dimethylethanolamine and adjusted to a 20% solidscontent, producing a core-shell type composition EM-1 according to theinvention. In this way, there were prepared nine specimens EM-1 to EM-9among which specimens EM-7 to EM-9 are for comparison.

                                      TABLE 3                                     __________________________________________________________________________                     Specimen                                                                      EM-1                                                                              EM-2                                                                              EM-3                                                                              EM-4                                                                              EM-5                                                                              EM-6                                                                              EM-7                                                                              EM-8                                                                              EM-9                         __________________________________________________________________________    Step (I)                                                                      First Pre-emulsion (*1)                                                       γ-Methacryloxy propyl trimethoxy                                                         2   4       2   4       2                                    silane                                                                        Vinyl trimethoxy silane  2           2                                        Styrene          20  20  20  20  20  20  20  20                               Methyl methacrylate                                                                            20  23  25  23  20  23  20  20  38                           Butyl acrylate   43  43  40  40  43  45  43  43  43                           Hydroxypropyl methacrylate                                                                     15  10  13  15  13  10  15  15  15                           Allyl acrylate                               2                                Ethylene glycol dimethacrylate                   2                            MGA (*2)                                         2                            Ammonium salt of polyoxyethylene                                                               1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5                          nonylphenol ether sulfate                                                     Ammonium persulfate                                                                            0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4                          Deionized water  45  45  45  45  45  45  45  45  45                           Step (II)                                                                     γ-Methacryloxy propyl trimethoxy                                                         2   4       2   2   2                                        silane                                                                        Vinyl trimethoxy silane  2                                                    Allyl methacrylate                                                                             2   2   3   2   2   2                                        Step (III)                                                                    Second Pre-emulsion (*1)                                                      Acrylic acid     16      18  16  16      16  16  16                           Methacrylic acid     16              20                                       Styrene          12  12  12  12  12  12  12  12  12                           Methyl methacrylate                                                                            20  18  18  24  22  16  20  20  20                           Butyl acrylate   44  44  40  40  40  40  44  44  44                           Hydroxyethyl acrylate                                                                          8   10  12  8   10  12  8   8   8                            Ammonium salt of polyoxyethylene                                                               0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3                          nonylphenyl ether sulfate                                                     Ammonium persulfate                                                                            0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2                          Deionized water  12  12  12  12  12  12  12  12  12                           First Pre-emulsion/                                                                            80/20                                                                             80/20                                                                             80/20                                                                             70/30                                                                             80/20                                                                             90/10                                                                             80/20                                                                             80/20                                                                             80/20                        Second Pre-emulsion                                                           (Weight ratio of monomers) (*3)                                               Before neutralization                                                         Particle size (nm) (*5)                                                                        121 131 130 110 135 103 110 115 106                          pH               2.3 2.4 2.2 2.6 2.8 2.5 2.3 2.5 2.5                          After neutralization (*4)                                                     Particle size (nm) (*5)                                                                        124 132 135 118 138 110 110 113 108                          pH               7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5                          __________________________________________________________________________

The symbols *1-*5 used in Table 3 designate the same as in Table 1.

II-2 Preparation of Crosslinking Agents (1) H-1

A hydrophobic melamine resin A (*6) was placed into a container with astirrer in an amount to provide a solids content of 25 parts. Twentyparts of aqueous solution of acrylic resin (*7) was added. Then themixture was stirred by a stirrer rotating the agitating blade at 1,000to 1,500 rpm while 88.75 parts of deionized water was gradually added.Thereafter stirring was further continued for 30 minutes so that thehydrophobic melamine resin was coated with the water-soluble acrylicresin, giving an aqueous dispersion of crosslinking agent H-1 in theform of particles 0.11 μm in mean particle size.

(6*) Preparation of hydrophobic melamine resin A

Into a 2 l four-necked flask equipped with a thermometer, a stirrer anda reflux condenser were placed 126 parts of melamine, 225 parts of 80%p-formalin (product of Mitsui Toatsu Chemicals Inc.) and 592 parts ofn-butanol. The mixture was adjusted to a pH of 9.5 to 10.0 with a 10%aqueous solution of sodium hydroxide, and reacted at 80° C. for 1 hour.

Then 888 parts of n-butanol was added. Then the mixture was adjusted toa pH of 5.5 to 6.0 with a 5% aqueous solution of sulfuric acid andreacted at 80° C. for 3 hours. After completion of reaction, thereaction mixture was neutralized to a pH of 7 to 7.5 with a 20% aqueoussolution of sodium hydroxide and was concentrated under reduced pressureat 60 to 70° C. The concentrate was filtered to obtain a melamine resinA.

The analysis of the resin showed that the resin had a nonvolatilecontent of 80%, a solvent dilution ratio of 3.6 given using awater/methanol solvent mixture (ratio by weight of 35/65) (hereinaftersimply referred to as "solvent dilution ratio"), and a weight-averagemolecular weight of 800.

(*7) Preparation of aqueous solution of acrylic resin

Into a reactor were placed 60 parts of butyl cellosolve and 15 parts ofisobutyl alcohol. The mixture was heated to 115° C. in a nitrogenstream. After the temperature reached 115°, there was added over a3-hour period a mixture of 26 parts of n-butyl acrylate, 47 parts ofmethyl methacrylate, 10 parts of styrene, 10 parts of 2-hydroxyethylmethacrylate, 6 parts of acrylic acid and 1 part of azoisobutyronitrile.After addition, the mixture was aged at 115° C. for 30 minutes. Amixture of 1 part of azobisisobutyronitrile and 115 parts of butylcellosolve was added over a 1-hour period. After aging for 30 minutes,the mixture was filtered through 200-mesh nylon cloth at 50° C.

The obtained reaction product had an acid value of 48 and a viscosity ofZ₄ (Gardner-bubble viscometer), and a nonvolatile content of 55%. Theproduct was neutralized with dimethylethanolamine to an equivalent anddeionized water was added, producing a 50% aqueous solution of acrylicresin WA.

(2) H-2

The same procedure as in H-1 was repeated with the exception of using acommercially available hydrophobic melamine resin B ("SuperbeckaminL-127-75B", product of Dainippon Ink and Chemicals, Inc., nonvolatilecontent of 80%, solvent dilution ratio of 0.5, and weight-averagemolecular weight of 1,400 to 1,800) in place of the hydrophobic melamineresin A used in H-1, giving an aqueous dispersion of crosslinking agentH-2 in the form of particles 0.12 μm in mean particle size.

II-3 Preparation of Dispersions of Pigment

(1) P-1: Preparation of concentrated dispersion of aluminum pigment

Into a container with a stirrer were placed 23 parts of aluminum paste(65% metal content) and 25 parts of butyl cellosolve. The mixture wasstirred for 1 hour, providing a concentrated dispersion of aluminumpigment P-1.

(2) P-2

Into a container with a stirrer were placed 100 parts of titanium white,10 parts of butyl cellosolve, 30 parts of a 50% aqueous solution ofacrylic resin WA as obtained in the same manner as above in *7 and 40parts of water. The mixture was homogeneously mixed and dispersed by ahigh-speed stirrer, giving a concentrated white dispersion P-2.

II-4 Preparation of Transparent Top Coat Compositions (1) T-1

Copolymerization was conducted in xylene using 25 parts of styrene, 25parts of 2-ethylhexyl methacrylate, 36.5 parts of n-butyl methacrylate,12 parts of 2-hydroxyethyl acrylate, 1.5 parts of acrylic acid and 2.5parts of a polymerization initiator (α,α'-azobisisobutyronitrile),producing a solution of acrylic resin with a resin solids content of60%. The resin had a hydroxyl value of 58 and an acid value of 12.

This resin and "Uban 20SE" (n-butanol-modified melamine resinmanufactured by Mitsui Toatsu Chemicals Inc., nonvolatile content of60%, solvent dilution ratio of 0.4 and weight-average molecular weightof 3,000 to 4,000) were mixed together in a ratio of 75:25 (by weight ofsolids). The mixture was adjusted to a viscosity of 25 seconds (Ford cup#4/20° C.) with "Swasol #1500" (trademark for organic solventmanufactured by Cosmo Oil Co. Ltd.), giving a clear coating compositionT-1.

(2) T-2

Into a three-necked flask equipped with a stirrer and a reflux condenserwere placed 20 parts of styrene, 43 parts of methyl methacrylate, 18parts of n-butyl acrylate, 3 parts of ethyl acrylate, 13 parts ofglycidyl methacrylate, 3 parts of hydroxyethyl methacrylate and 100parts of toluol. After addition of 1.5 parts of benzoyl peroxide, themixture was heated until the temperature of contents reached 90 to 100°C. The mixture was maintained at this temperature for 3 hours and afurther 2 parts of benzoyl peroxide was added. The mixture was thenmaintained at the same temperature for 4 hours to complete thecopolymerization. The thus obtained copolymer was left to stand,solidified and pulverized by a crusher to 6 minus-mesh particles,producing an acrylic resin powder.

A 100-part portion of the acrylic resin powder was mixed with 13 partsof decanedicarboxylic acid and 1 part of coating surface improver(product of Mitsubishi Monsant Chemical Co., Ltd., "Modaflow"). Themixture was melted and kneaded at about 100° C. for 10 minutes by aheating roll. After cooling, the mass was finely divided to a particlesize of 20 to 100 μm, producing a clear coating composition T-2.

Examples 7 to 20 and Comparison Examples 4 to 12

Water-dispersible coating compositions of the invention and those forcomparison were preapred using the present compositions or comparisoncompositions, crosslinking agents and, optionally, dispersions ofpigment obtained above in Examples.

The water-dispersible coating compositions or those for comparisonhaving the solids contents as shown below in Table 4 were prepared byhomogeneously mixing and dispersing the present compositions orcomparison compositions, crosslinking agents and, optionally,dispersions of pigment and adding 2.9 to 2.1 parts of "Acrysol ASE-60"(trademark for a thickener manufactured by Rohm and Haas Co.) and 0.27to 0.20 part of dimethylethanolamine to adjust the mixture to anapparent viscosity of 3,000 cps/6 rpm (Brookfield viscometer) and a pHof 7.80.

                                      TABLE 4                                     __________________________________________________________________________                   Crosslinking                                                                           Pigment  Solids                                       Composition    agent    dispersion                                                                             content                                      Kind      Amount                                                                             Kind                                                                              Amount                                                                             Kind                                                                              Amount                                                                             (%)                                          __________________________________________________________________________    Ex.                                                                            7    EM-1                                                                              325  H-1 140  --  --   21.5                                          8    EM-2                                                                              325  H-1 140  --  --   21.5                                          9    EM-3                                                                              300  H-1 160  --  --   21.7                                         10    EM-4                                                                              300  H-2 160  --  --   21.7                                         11    EM-5                                                                              350  H-2 120  --  --   21.3                                         12    EM-6                                                                              350  H-2 120  --  --   21.3                                         Comp. Ex.                                                                      4    EM-7                                                                              325  H-1 140  --  --   21.5                                          5    EM-8                                                                              325  H-1 140  --  --   21.5                                          6    EM-9                                                                              325  H-1 140  --  --   21.5                                         Ex.                                                                           13    EM-1                                                                              350  H-1 120  P-1  42  22.1                                         14    EM-2                                                                              350  H-1 120  P-1  42  22.1                                         15    EM-5                                                                              300  H-2 160  P-1  55  22.7                                         16    EM-6                                                                              300  H-2 160  P-1  55  22.7                                         Comp. Ex.                                                                      7    EM-7                                                                              350  H-1 120  P-1  42  22.1                                          8    EM-8                                                                              350  H-1 120  P-1  42  22.1                                          9    EM-9                                                                              350  H-1 120  P-1  42  22.1                                         Ex.                                                                           17    EM-1                                                                              300  H-1 160  P-2 181  33.6                                         18    EM-2                                                                              300  H-1 160  P-2 181  33.6                                         19    EM-3                                                                              350  H-2 120  P-2 181  33.1                                         20    EM-4                                                                              350  H-2 120  P-2 181  33.1                                         Comp. Ex.                                                                     10    EM-7                                                                              300  H-1 160  P-2 181  33.6                                         11    EM-8                                                                              300  H-1 160  P-2 181  33.6                                         12    EM-9                                                                              300  H-1 160  P-2 181  33.6                                         __________________________________________________________________________

Examples 21 to 28 and Comparison Examples 13 to 18

Coating films were each formed from the water-dispersiblepigment-containing coating compositions (base coat compositions)prepared above in Examples 13 to 20 and Comparison Examples 7 to 12,respectively and the transparent top coat compositions by the two-coatone-bake method.

Substrates to be coated were prepared by the following method. A steelpanel (7.5×15×0.2 cm) was surface-treated with "BONDERITE #3030"(trademark for zinc sulfate type primer manufactured by NihonParkerizing Co., Ltd.). The primed panel was coated with "ELECTRON No.9200" (trademark for epoxy-type cationic electrodeposition coatingcomposition manufactured by Kansai Paint Co., Ltd.). Then the coatedpanel was coated with "Amilac N-2 Sealer" (trademark for aminopolyesterresin-type intercoat composition manufactured by Kansai Paint Co.,Ltd.).

Table 5 below shows the the kind and other specific conditions ofwater-dispersible coating compositions and transparent top coatcompositions.

The applications were carried out in a manner as described below.Deionized water was added to the water-dispersible coating compositionas the base coat composition to a solids content of 20%. The dispersionwas adjusted to a viscosity of 1,000 to 2,000 cPs with a thickener andapplied to the substrate at 25° C. and RH of 70 or 90% by electrostaticspraying to a thickness as shown in Table 5. Then the coated substratewas heated at room temperature or 100° C. or lower until the watercontent of coating reached 10% by weight or less. The transparent topcoat composition was applied to the coated substrate by electrostaticspraying and the two coatings were concurrently cured by heating. Table5 below shows the thickness of top coating and the heating conditionsfor curing.

                  TABLE 5                                                         ______________________________________                                        Base coating      Top coating   Heating                                                  Film              Film     (°C.-                            Kind       Thickness (μ)                                                                         Kind   Thickness (μ)                                                                       min.)                                   ______________________________________                                        Ex.                                                                           21       7     13-15      T-1  40       140-30                                22       8     13-15      T-2  50       160-30                                23       9     13-15      T-1  40       140-30                                24      10     13-15      T-2  50       160-30                                25      11     25-30      T-1  40       140-30                                26      12     25-30      T-1  40       140-30                                27      13     25-30      T-2  50       160-30                                28      14     25-30      T-2  50       160-30                                Comp. Ex.                                                                     13       4     13-15      T-1  40       140-30                                14       5     13-15      T-1  40       140-30                                15       6     13-15      T-1  40       140-30                                16       7     25-30      T-1  40       140-30                                17       8     25-30      T-1  40       140-30                                18       9     25-30      T-1  40       140-30                                ______________________________________                                    

(Note)

1. The kinds of base coat compositions are shown in Table 5 with thenumbers of Examples or Comparison Examples appearing in Table 4.

2. The thicknesses indicated in Table 5 are those of the cured coatingfilms.

III. RESULTS OF PERFORMANCE TEST

Performance tests were conducted using the water-dispersible coatingcompositions prepared in Examples 7 to 12 and Comparison Examples 4 to6, base coat compositions used in Examples 21 to 28 and ComparisonExamples 13 to 18, and coatings formed from these coating compositions.The water-dispersible coating compositions prepared in Examples 7 to 12and Comparison Examples 4 to 6 were each applied by spraying toaforesaid substrates to a thickness of 15 μm (when cured) and the coatedsubstrates were heated at 140° C. for 30 minutes to obtain cured coatingfilms. The results are shown below in Table 6.

The test methods are as follows.

Sagging

To check sagging, the substrate with a hole, 1 cm in diameter, wasperpendicularly disposed. The base coat composition and the top coatcomposition were applied to the substrate in the same manner as aboveand cured. The length (mm) of sagging of the applied composition runningdownward from the hole was measured. The top coat composition was notapplied in testing the compositions of Examples 7 to 12 and ComparisonExamples 4 to 6.

Mottling

The substrate was coated with metallic coating in the same manner as inthe sagging test. The coated substrate was observed to check themetallic mottling. In Table 6, the mark A represents no or littlemottling, the mark B a high degree of mottling, and the mark C asignificant degree of mottling.

Surface smoothness

The coating surface was inspected with the unaided eye to evaluate thesurface smoothness. In Table 6, the mark A shows a good surfacesmoothness, the mark B a poor one and the mark C a markedly poor one.

Adhesion

The coated substrate was cut crosswise with a cutter knife to thesubstrate so that parallel cuts are provided in a checkerboard-likepattern with a spacing of 1 mm between adjacent cuts to produce 100squares (each 1 cm²). An adhesive cellophane tape was adhered to the cutcoating surface, and the tape was peeled off with a strong force toevaluate the adhesiveness. In Table 6, the mark A represents no peeling,the mark B a little peeling between the base coating and the top coatingand the mark C a marked degree of peeling between the coatings.

Gloss

The gloss was determined in terms of specular gloss (60°).

Water resistance

The coated substrate was immersed in water at 40° C. for 360 hours afterwhich the coating surface was visually inspected. In Table 6, the mark Ashows no undesired change, the mark B a little blistering and the mark Ca marked degree of blistering.

Storability

The storability was expressed in a ratio (times) of increased viscosityto the initial viscosity (cps/6 rpm, Brookfield viscometer) in respectof the water-dispersible coating compositions stored at 40° C. for 20days.

Mechanical stability

The mechanical stability is the result obtained by testing singly thewater-dispersible coating compositions prepared in the Examples andComparison Examples, and was evaluated in terms of viscosity stabilityand TI value-retaining percentage (to be clarified below). The viscositystability was given by the following procedure. A viscosity (η₀) wasdetermined by Brookfield viscometer (rotating the agitating blade at 6rpm) after the composition was left to stand for 24 hours at roomtemperature. Thereafter the composition was stirred at room temperaturefor 3 minutes by a stirrer (rotating the agitating blade at about 800rpm). Then after standing for 5 minutes, a viscosity (η₁) was measuredin the same manner as above. The ratio of η₁ /η₀ was calculated topresent the viscosity stability. The TI value-retaining percentage wasprovided by the following method. After the composition was left tostand at room temperature for 24 hours, viscosities η₆ and η₆₀ weredetermined by Brookfield viscometer rotating the agitating blade at 6rpm and 60 rpm, respectively and a viscosity ratio TI₀ (=η₆ /η₆₀) wascalculated. Subsequently the composition was stirred at room temperaturefor 3 minutes by a stirrer rotating the agitating blade at about 800rpm. After standing for 5 minutes, viscosities η₆ and η₆₀ were measuredby Brookfield viscometer rotating the agitating blade at 6 rpm and 60rpm, respectively to obtain a viscocity ratio TI₁ (=η₆ /η₆₀). The TIvalue-retaining percentage was obtained by the following equation: (TI₁/TI₀)×100 (%).

                                      TABLE 6                                     __________________________________________________________________________    Relative        Relative                          Mechanical                  humidity 70%    humidity 90%                                                                            Surface        Water                                                                              Stora-                                                                            stability                   Sagging    Mottling                                                                           Sagging                                                                            Mottling                                                                           smoothness                                                                          Adhesion                                                                           Gloss                                                                             resistance                                                                         bility                                                                            (A)                                                                              (B)                      __________________________________________________________________________    Ex.                       A                                                    7    1.2       2.5       A     A    100      1.0 1.0                                                                              95                        8    1.0       2.3       A     A    100      1.1 1.0                                                                              92                        9    0.9       2.1       A     A     99      1.1 1.0                                                                              93                       10    1.4       2.8       A     A    100      1.0 1.0                                                                              92                       11    1.1       2.3       A     A     99      1.2 1.0                                                                              94                       12    1.3       2.7       A     A    100      1.1 1.0                                                                              92                       Comp. Ex.                                                                      4    4.1       6.0       A     A    100      1.2 0.85                                                                             67                        5    4.3       6.3       A     A    100      1.3 0.87                                                                             68                        6    4.0       6.2       A     A    100      1.3 0.88                                                                             69                       Ex.                                                                           21    2.1  A    3.3  A    A     A    100 A    1.0 0.98                                                                             90                       22    2.0  A    3.2  A    A     A     98 A    1.1 0.97                                                                             89                       23    2.0  A    3.0  A    A     A    103 A    1.2 1.0                                                                              93                       24    2.2  A    3.4  A    A     A     98 A    1.1 0.98                                                                             80                       25    3.2       3.8       A     A     98 A    1.2 0.99                                                                             92                       26    3.1       3.7       A     A     97 A    1.2 0.98                                                                             90                       27    3.0       3.5       A     A     96 A    1.1 1.0                                                                              93                       28    3.1       3.8       A     A     96 A    1.2 0.98                                                                             91                       Comp. Ex.                                                                     13    6.0  B    15.4 B    A     A    100 A    1.2 0.81                                                                             60                       14    6.2  B    20.3 B    A     A    100 A    1.3 0.80                                                                             63                       15    6.1  B    18.2 B    A     A    100 A    1.3 0.82                                                                             64                       16    7.2       20.1      A     A     95 A    1.4 0.84                                                                             72                       17    7.5       23.4      A     A     93 A    1.5 0.86                                                                             74                       18    7.3       21.9      A     A     94 A    1.4 0.87                                                                             76                       __________________________________________________________________________

We claim:
 1. A water-dispersible resin composition prepared by a processcomprising:(I) reacting a silane monomer (A) which is at least onemonomer selected from the group consisting of vinyltrimethoxysilane,vinyltriethoxysilane, acryloxyethyltrimethoxysilane,methacryloxyethyltrimethoxysilane, acryloxypropyltrimethoxysilane,methacryloxypropyltrimethoxysilane, acryloxypropyltriethoxysilane,methacryloxypropyltriethoxysilane and vinyltris (β-methoxyethoxy) silanewith a vinyl monomer (B) in an aqueous medium to obtain athree-dimensionally crosslinked particular polymer, (II) reacting theobtained particular polymer with the silane monomer (A) or a mixture ofthe silane monomer (A) and an allyl acrylate or allyl methacrylate (C),and (III) copolymerizing the resulting reaction product with a vinylmonomer component comprising a carboxyl-containing vinyl monomer (D),followed by neutralizing the carboxyl group whereby a particular polymerhaving a core/shell structure is obtained, wherein the monomers are usedin steps (I) and (III) in the following proportions, based on thecombined weight of all the monomers used in steps (I) and (III); about60 to about 90% by weight in step (I); and about 40 to about 10% byweight in step (III).
 2. A composition according to claim 1 whereinabout 0.5 to about 20% by weight of the silane monomer (A) and about99.5 to about 80% by weight of the vinyl monomer (B) are used in thestep (I) based on the combined weight of these two monomers.
 3. Acomposition according to claim 1 wherein about 1 to about 50% by weightof the carboxyl-containing vinyl monomer (D) included in the vinylmonomer component is used in the step (III) based on the weight of thevinyl monomer component.
 4. A composition according to claim 1 whereinabout 0.5 to about 2 moles of the silane monomer (A) is used in the step(II) per mole of the silane monomer (A) used in the step (I).
 5. Acomposition according to claim 1 wherein about 0.5 to about 5 parts byweight of the allyl acrylate or allyl methacrylate (C) is used in thestep (II) per 100 parts of the particulate polymer obtained in the step(I).